It is well known to use steam injection to reduce NOx emissions from gas turbine engines and/or to boost their powers. United Kingdom patent application no. 2219070 discloses a dual fuel injector provided with a steam injector to inject steam into a gas turbine combustor when the engine is operating on liquid fuel. It should be noted that steam can also be injected when the engine is operating on gas fuel only. When the engine is operating on liquid fuel a proportion of delivery air from the compressor of the engine is allowed to flow through purge holes in the fuel injector to provide a positive flow of air through the gas nozzles in order to prevent these nozzles becoming partially or wholly blocked by carbon deposits which can accumulate when operating on liquid fuel only.
However, the steam is injected into the compressor delivery flow upstream of the fuel injector and thus it is possible for a mixture of steam and air to enter the gas fuel passages. These passages comprise a small gas fuel manifold within the fuel injector, a large outer gas fuel manifold around the engine, supply ducts from the outer gas manifold to the fuel injectors, and a gas fuel supply pipe to the outer manifold. When the engine operates on gas fuel, the fuel pressure is higher than that of the compressor delivery air and therefore obviates any air flow into the gas passages, but when operating on liquid, fuel, the steam will be at higher pressure than the gas passages and will therefore enter them. On contacting the relatively cold air in the gas fuel supply system, the steam will condense and the condensate will collect in the gas fuel passages. If an attempt is made to initiate a fuel transfer to gas fuel, or the engine is shut down and restarted on gas fuel, condensate which may have collected in the gas fuel supply passages may be injected into the engine through the gas fuel nozzles in the fuel injector when the gas fuel supply valve is opened. This may have an undesirable effect on the combustion process.